Non-contact platforms for the conveyance of thin flat objects have many applications. Examples of such applications include conveying silicon substrates or glass panes during the fabrication of flat panel displays (FPD) or solar cells. During the fabrication process, the object may be transported to various locations for the performance of such operations as inspection, cleaning, coating, heating, and etching. A typical non-contact conveyance system may include a precisely formed rigid surface on which are distributed sources or ports of injected pressurized fluid (as described by Levin et al. in U.S. Pat. No. 6,523,572), of vacuum or suction (described by Levin et al in U.S. Pat. No. 6,644,703), or of both pressurized fluid and vacuum (Yassour et al. in US 2006/0054774), all of which are incorporated herein by reference. Ports through which fluid is injected or evacuated may include self adaptation segmented orifice (SASO) nozzle devices, as described by Levin et al. in WO 01/14782, and also in U.S. Pat. No. 6,523,572, U.S. Pat. No. 6,644,703, and US 2006/0054774, all incorporated herein by reference. A SASO nozzle includes a fluid conduit with a plurality of fins mounted on the internal wall of the conduit. The fins are arranged on opposite sides of the conduit. Each fm mounted on one side of the conduit is positioned opposite a space between two fins on the opposite side. The fins serve to increase the fluid resistance of the conduit, reducing flow for a given applied fluid pressure.
The rigid surface may include a single contiguous platform that supports the entire area of the substrate, several separate parallel rails, or a combination of contiguous platforms and rails. Such a non-contact conveyance system as adapted to a heating application is described by Yassour et al. in US 2008/0145190, which is incorporated herein by reference.
The distribution of pressure and vacuum ports may be continuous. For example, the rigid surface may be porous rigid surface through which pressurized fluid is injected. Alternatively, the distribution may be discrete, for example, by distributing pressure and vacuum nozzles over the rigid surface with a particular distribution. When a thin flat object, such as a substrate, is placed on the rigid surface with a pressure-vacuum (PV) or pressure supply, a thin cushion of fluid is created between the object and the rigid surface. The fluid may be gas or liquid. The fluid cushion produced prevents the object from contacting the rigid surface, and may tend to hold the object at an approximately fixed distance from the surface.
A continuous supply of fluid, quantified by the mass flow rate (MFR) of the fluid, may be required during the conveying process. The continuous supply of fluid may maintain the pressure field between the object and the rigid surface. Typical values of the thickness (c) of the cushion of fluid may vary from about 1 μm to about 2 mm. The required MFR to maintain a given fluid cushion pressure field is proportional to ε3. Therefore, a fluid cushion thickness may be limited by practical limitations on the MFR to a range of 20 μm to 200 μm for a PV configuration, and 60 μm to 600 μm for a pressure-only configuration.
A uniform ε and a uniform supporting force on the object may be achieved by uniformly distributing the pressure and vacuum supply over the area of the rigid surface. However, a uniform fluid cushion may not necessarily be advantageous. A thin flat object with a large area may be somewhat flexible. The mechanical stiffness of the object may not be sufficient to maintain its shape when supported by a uniform pressure field. Placement of such a flexible substrate on a fluid cushion with a predetermined pressure field distribution may result in large scale deformation of the object. Deformation of the object may lead, in turn, to non-uniform application of a process being performed on the object. Such a process may include, for example, heat transfer, cleaning, or etching. In order to reduce or prevent such deformation, evacuation slits may be located on the rigid surface to assist in configuring the fluid cushion to counteract any tendency of the object to bend or deform. The evacuation slits may be arranged, for example, parallel to a motion path of the object. The fluid pressure in the evacuation slits may be at, or close to, atmospheric pressure. For example, ports along the evacuation slits may open to the surrounding atmosphere or to a source at a low vacuum pressure.
However, prevention of large scale deformation may not be sufficient in the presence of local deformation of the object. For example, near the edges or corners of a flat object, edge effects in the fluid cushion near the borders of the object may lead to deflection or bending of edges or corners of the object. In addition, a thin flat object supported by a fluid cushion pressure field may be subject to undesirable dynamic phenomena. For example, a pneumatic hammer effect may result from an instability caused by an interaction of the compressibility of a gaseous fluid and mechanical vibration of the object. In particular, such an effect may occur where there is resonance between natural vibration frequencies of the object and of the fluid cushion.
Deflection of an edge or corner of the object may be especially undesirable in the vicinity of obstacles in path along which the object is conveyed. Such an obstacle may include a gap in the rigid surface and fluid cushion, or a protruding projection, such as a drive wheel (as described by Yassour in US 2008/0302637).
A gap in the fluid cushion may result from a gap in the rigid surface, such as gaps between nearby sections of a conveyance system. If the gap is small, an object may be conveyed across the gap despite bending of the object. However, in some applications, for example inspection, a wide gap may be required. The leading edge of an object being conveyed across a wide gap may be deflected downward by the force of gravity. The amount of the deflection may depend on such factors as the elastic modulus of the object, its moment of inertia, its speed, and the width of the gap. In the event that the amount of deflection of the leading edge is more than ε, the width of the fluid cushion, the leading edge may contact or strike the rigid surface. Contact with the rigid surface may cause damage to the object, to the surface, or to both. On the other hand, when an obstacle protrudes out of the surface and the leading edge of the object is not deflected sufficiently to avoid the obstacle, the object may collide with the obstacle.
For example, a typical FPD material may be a sheet of glass with a thickness of 0.7 mm. A typical value of ε may be, for example, about 100 μm. A gap that such a sheet of glass may cross without colliding with the surface may be limited to a width of about 80 mm, based on a simplified calculation. With a wider gap, the leading edge of the glass sheet may bend toward the surface by more than ε, and may collide with a rigid surface. In practice, dynamic phenomena and edge effects may significantly increase the amount of bending and decrease the size of the maximum allowable gap.
Therefore, there is a need for a non-contact platform that can reduce deformation of a thin object being conveyed, and that avoids collision of the object with the edges of gaps or other obstacles.
It is an object of the present invention to provide a system and method for controlling the support of a thin object by a non-contact platform so as to reduce deformation of the object, and to safely convey the object over gaps and obstacles.
Other aims and advantages of the present invention will become apparent after reading the present invention and reviewing the accompanying drawings.